The behaviour of polyphosphate accumulating acinetobacter isolates in an anaerobic–aerobic chemostat

The presence of a specialized physiological group, the so-called “poly-P bacteria”, in activated sludge has been proposed to explain the uptake of organic carbon sources under anaerobic conditions and the cyclic release and uptake of phosphate in activated-sludge plants showing enhanced biological phosphate removal (EBPR). Members of the genus Acinetobacter have been considered to play a role in these processes, although their importance has been questioned. Indeed, the ability of Acinetobacter to alternately accumulate polyhydroxyalkanoates under anaerobic conditions and polyphosphate under aerobic conditions has not been demonstrated. In this study, under aerobic batch-culture conditions, two strains of Acinetobacter which had been isolated from treatment plants exhibiting EBPR were shown to accumulate high levels of polyphosphate or a “lipid” reserve which in one strain was identified as a polyhydroxyalkanoate. When cultured in a chemostat which allowed cyclic exposure to anaerobic and aerobic conditions, neither of the two strains showed release of phosphate or uptake of acetate under anaerobic conditions. These findings suggest that the behaviour of these strains does not conform to the usually accepted models for poly-P bacteria in EBPR systems. Nevertheless, strains of Acinetobacter which accumulate high levels of polyphosphate may still contribute to phosphate removal in EBPR systems.